These reinforcing plies come in the form of a set of portions of thread, of given length, coated in a rubber compound, laid parallel to one another and making a given angle with the longitudinal direction of the ply.
During the tire-building operation it is necessary to cut portions of ply the length of which is tailored to the circumference of the form intended to accept this component. Hence, upstream of the tire-building machine, there is a cutting means able to separate a portion of ply off from the continuous strip of ply, by cutting through the bridge of rubber compound between two adjacent threads.
As a general rule, this cutting means moves along a straight support the orientation of which corresponds to a theoretical angle formed by the threads with the longitudinal direction.
However, it is found to be the case that the actual angle of the threads with the longitudinal direction differs by one or two degrees of angle from the theoretical angle that the creation of the ply sought to achieve.
It therefore follows that the cutting means penetrating the space between two threads and following a path corresponding to the theoretical angle strikes the threads transversely and causes the ply to become deformed or damaged.
Hence, cutters known as floating cutters have been developed, these being supported by an articulated frame and able to move freely in a direction perpendicular to the direction for support. The threads adjacent to the cutting line are then used to guide the cutter transversely to its direction of travel. This device is described by way of example in publication U.S. Pat. No. 4,857,123.
Alternatively, publication JP 3 251 440 describes a device comprising a measurement unit formed of magnetic means to control the transverse deviation of the cutter, but this means is suited only to plies formed of metal reinforcing threads.
However, where there is a desire to cut plies containing threads of small diameter and low rigidity, it is found that the threads no longer act as guides because of their poor mechanical integrity. Indeed, for the device known from the prior art to work correctly, it is necessary for the threads to be able to withstand the transverse force needed to cause the transverse movement of the cutting means. Further, it is found that the greater the force applied to the cutter in order for it to be capable of producing the desired effect, namely of cutting the ply, the greater this transverse force becomes. This phenomenon is particularly keenly felt when the cutting means is cutting through the two transverse ends of the ply, and lifting or folding of the ply in this region is not uncommon.